1. Field of the Invention
The present invention relates to a semiconductor device wherein a semiconductor substrate is hermetically sealed with a cap.
2. Description of the Related Art
A capacitive acceleration sensor, for example, of the prior art has been made in such a constitution as an acceleration sensor element 1 formed on a semiconductor substrate 3 is hermetically sealed with a cap 5 so that a space is formed over the element 1, as shown in FIG. 6. The semiconductor substrate 3 and the cap 5 are generally bonded with each other by bringing polysilicon layer formed around the semiconductor substrate 3 and an Ni layer 4 formed around the cap 5 into contact with each other, and heating to a temperature above the eutectic temperature of Ni and silicon. With this bonding method, since the polysilicon layer 23 formed on the semiconductor substrate 3 for the purpose of bonding is formed simultaneously with the polysilicon layer which is formed as a conductor for the acceleration sensor element, the polysilicon layer is doped with an impurity of phosphorus. As a result, there has been such a problem that direct bonding of the polysilicon layer 23 and Ni is impeded by the diffusion of phosphorus into Ni caused by the heat generated during bonding.
Therefore, the bonding method of the prior art has been employing such a process as an insulation film 22awhich prevents the diffusion of phosphorus is formed on the phosphorus-doped polysilicon layer 23 that has been formed simultaneously with the polysilicon layer of the acceleration sensor element, then a polysilicon layer 21 which is not doped with phosphorus is formed on the insulation film 22a, then the polysilicon layer 21 is bonded with the Ni layer 4.
With the method of the prior art, however, the cap 5 is isolated from ground (not grounded) since the phosphorus-doped polysilicon layer 23 and the polysilicon layer 21 which is not doped with phosphorus are electrically insulated from each other by the insulation film 22a. Thus there has been such a problem that the acceleration sensor is prone to external noise such as electrostatic or electromagnetic interference.
Accordingly, such measures has been taken in the prior art as a metal film 6 is formed on the cap 5 with the metal film being connected to an external ground conductor by means of wiring so as to ground the cap, as shown in FIG. 6, thus providing protection against the external noise such as electrostatic or electromagnetic interference.
It is therefore an object of the present invention to provide a semiconductor device which has a bonding layer capable of providing electrical continuity between the cap and the semiconductor substrate and provides good bonding.
In order to achieve the object described above, the semiconductor device of the present invention comprises a semiconductor substrate whereon an element is formed on one of the principal planes thereof and a cap which hermetically seals the element so that a space is formed over the element, while the element is sealed by bonding a laminated bonding layer, which is formed around the element provided on one of the principal planes of the semiconductor substrate, and an Ni layer formed on the cap, wherein the laminated bonding layer is constituted from a first polysilicon layer which is doped with an impurity, an insulation layer and a second polysilicon layer which is not doped with an impurity being formed one on another, while the first polysilicon layer and the second polysilicon layer contact with each other in a part thereof so that the impurity diffuses through the contact area from the first polysilicon layer into the second polysilicon layer.
In the semiconductor device of the present invention having such a constitution as described above, since a current path is formed in the semi-insulating second polysilicon layer which is not doped with the impurity, by the impurity which has diffused from the first polysilicon layer, electrical continuity can be established between the first polysilicon layer which has electrical conductivity formed on the semiconductor substrate and the Ni layer of the cap.
In the semiconductor device of the present invention, in order to achieve reliable continuity between the first polysilicon layer and the cap, it is desirable that the first polysilicon layer and the second polysilicon layer contact with each other at two or more separate points.
Also in the semiconductor device of the present invention, in order to reduce the resistance of the current path between the first polysilicon layer and the cap, it is more desirable that the laminated bonding layer is provided so as to surround the element and the first polysilicon layer and the second polysilicon layer contact with each other through a ring-shaped contact area provided on the inner or outer circumference of the laminated bonding layer.
In case the element of the semiconductor device of the present invention is constituted to include a polysilicon layer, this polysilicon layer and the first polysilicon layer can be formed in the same process.
In the semiconductor device of the present invention, the cap can be formed from silicon.